The present invention pertains to the field of optical fibers. More particularly, the present invention pertains to an optical fiber, including a pure silica core, having a Bragg grating formed in the core.
According to the prior art, a Bragg grating in an optical fiber is usually created in the core region of the optical fiber, a core region typically of silica and containing germanium or other dopants so as to impart to it, by exposure to ultraviolet (UV) light, the photo-refractive structure known as a Bragg grating. See, e.g. K. O. Hill et al., xe2x80x9cPhotosensitivity In Optical Fiber Waveguides: Application To Reflection Filter Fabricationxe2x80x9d, Appl. Phys. Lett. 32, 1978, pp. 647-649; and G. Meltz et al., xe2x80x9cFormation Of Bragg Gratings In Optical Fibers By A Transverse Holographic Methodxe2x80x9d, Opt. Lett. 14, 1989, pp. 823-825.
Bragg gratings are generally produced in such a doped silica core of an optical fiber by laterally exposing the optical fiber to a three dimensional fringe pattern created by interfering holographically two coherent high intensity UV beams or by exposing the optical fiber to UV light passed through a diffractive optical element called a phase mask. The doped glass, by virtue of the lattice defects (in this case point imperfections) associated with the dopants, interacts with the bright portions of a UV pattern to produce light-absorbing color centers. Either technique produces a pattern of UV light consisting of alternating bright and dark regions. The doped glass interacts with the bright portions of the UV pattern in such a way that its refractive index is modified leaving a refractive index modulated according to the UV pattern.
Bragg gratings have been demonstrated in commercial telecommunications-grade optical fibers that contain germanium in the core (Corning SMF-28 for example). However, high levels of photosensitivity of these optical fibers is needed to achieve high reflectivity gratings and compatibility with manufacturing processes within reasonable exposure parameters. This has led to the use of hydrogenation, along with the development of special optical fibers containing high levels of germanium or other photosensitizing dopants, as a means to increase the number of defect centers to promote optical fiber photosensitivity. Pure silica core optical fibers, which contain little or no such defects, have been found unsuitable as a host material for forming Bragg gratings using UV exposure processes.
Pure silica core optical fibers however, are attractive in a number of applications due to their superior resistance to hydrogen-induced attenuation and to nonlinear effects. The hydrogen-induced attenuation is attributed to changes over time of energy levels of the doped glass structure; it is well known that UV-induced refractive index changes in xe2x80x9ccolor centerxe2x80x9d Bragg gratings decay over time, at a rate depending on temperature, corresponding to the thermal depletion of lower energy trap states. (See, e.g. T. Erdogan et al., xe2x80x9cDecay Of Ultraviolet-Induced Bragg gratingsxe2x80x9d, Appl. Phys. Lett. 76, 1994, pp. 73-80.) Accelerated aging techniques (thermal annealing) are routinely employed as a means of providing stable grating performance. But if a Bragg grating could be provided in pure silica, aging would not be necessary. What is needed is a way of providing a Bragg grating in a pure silica core of an optical fiber, which would then not suffer from the drawbacks of Bragg gratings as provided by the prior art, namely by relying on doping the core.
The present invention builds on another approach besides accelerated aging to provide a stable Bragg grating. It forms a Bragg grating by providing in a target length of an optical fiber a periodic depletion (a sequence of alternating high and low concentrations) of an index-varying dopant in the core within the target length, thus providing an index modulation and corresponding Bragg resonance. As opposed to the xe2x80x9ccolor centerxe2x80x9d type gratings whose index modulation is subject to decay across a wide temperature range, the gratings of the invention are stable to temperatures in excess of 500xc2x0 C., where the glass network becomes mobile. (See M. A. Fokine et al., xe2x80x9cHigh Temperature Resistant Bragg Gratings Fabricated In Silica Optical fibersxe2x80x9d, ACOFT, 1996.)
These gratings, called chemical gratings (as opposed to color center gratings formed by exposure of doped silica to UV light), rely on the presence of (point) defect sites that lead to hydroxyl formation. Chemical gratings are formed in a three-step process:
In the first step, a special fluorine-codoped germanium silicon (Gexe2x80x94Si) single-mode optical fiber is hydrogenated. The hydrogen loaded optical fiber then undergoes a typical UV exposure used to write traditional Bragg gratings, i.e. it is exposed to an interference or diffraction pattern of UV light. In those portions of the optical fiber exposed to UV light, as opposed to those portions exposed to only low intensity UV light (or to no UV light at all), the UV light causes a photochemical reaction in which the free hydrogen migrates to defect sites (primarily from the Ge dopant) and forms hydroxyl. The optical fiber is then heated causing atomic fluorine (F) in the presence of hydroxyl (OH) to form (volatile) hydrogen fluoride (HF), which then thermally diffuses rapidly out of the optical fiber. Because the optical fiber is exposed to spatially varying amounts of UV radiation, the hydroxyl formation, and thus HF concentration, will also vary according to the UV exposure pattern. Therefore, the fluorine depletion from the out-diffusion of HF will follow the pattern, leading to a refractive index modulation where fluorine depleted sections exhibit a higher refractive index (by virtue of fluorine being an index-lowering dopant). This reaction can be observed by monitoring the optical spectrum of the grating during these steps. After the UV exposure, a xe2x80x9cfirstxe2x80x9d grating appears. Upon the heating step and the subsequent chemical reaction, the xe2x80x9cfirstxe2x80x9d grating is erased prior to the formation and appearance of the xe2x80x9cchemicalxe2x80x9d grating.
Fluorine in low concentrations is commonly used by the optical fiber industry as an index lowering dopant in providing cladding glasses. Recently, chemical gratings have been formed in commercial optical fibers, by thermally diffusing the fluorine present in the optical cladding region into the core region of the optical fiber. Applying this diffusion technique can eliminate the requirement for a special fluorine-doped core optical fiber. A process for making chemical gratings in commercial optical fibers with fluorinated cladding regions has been demonstrated ( M. A. Fokine and R. Stubbe, Private Correspondence, Sep. 30, 1998) that is identical to the original process, except for the addition of a fluorine diffusion step, as shown below.
In this modified process, a commercial Ge-doped optical fiber is used. The presence of the Ge dopant provides a suitable population of defect sites to form hydroxyl in the presence of hydrogen, during UV exposure.
This process, without more, is not suitable for providing chemical gratings in pure silica core optical fibers because of the lack of a suitable number of defect sites in pure silica. What is needed is a way of providing a suitable number of defect sites in pure silica, without doping, and so adapting a pure silica material so that it is suitable for writing a Bragg grating that would then be stable (against aging) because of not relying on dopants in the core.
Accordingly, the present invention provides a method for adapting an optical fiber that has a substantially pure silica core, so as to enable imprinting a Bragg grating in the core, and an optical fiber having the properties that result from the method. The method includes the steps of: providing an optical fiber having a silica core substantially free of dopants, and having a fluorine-bearing cladding; selecting a length of the optical fiber; performing a fluorine in-diffusion step by heating the selected length of optical fiber so as to influence the fluorine in the cladding to diffuse into the core and bond with silicon in the core; performing a defect creation step throughout the length of the optical fiber, by causing changes in the silicon-fluorine bond; performing a step of hydrogenation of the length of the optical fiber so as to introduce hydrogen into the core; exposing the length of the optical fiber to a pattern of ultraviolet (UV) light alternating in high and low intensity, so as to promote forming hydroxyl bonded to silicon in the parts of the length exposed to the high intensity UV light; and heating the length of the optical fiber so as to cause a reaction producing hydrogen fluoride and to volatilize the hydrogen fluoride. The method creates in the core of the optical fiber alternating regions of relatively high and relatively low index of refraction. Thus, the method creates an optical fiber having a silica core substantially free of dopants except in a selected length; a fluorine-bearing cladding; and the selected length of the optical fiber bearing atomic fluorine in a concentration that varies periodically along the selected length.
In a further aspect of the invention, the defect creation step is performed by heating the length of optical fiber and exposing it to a chemical agent that is either an oxidizing agent or a reducing agent.
In another aspect of the invention, the defect creation step includes performing hydrogen flame brushing.
In yet another aspect of the invention, the defect creation step is performed by exposing the length of the optical fiber to ionizing radiation.
The present invention yields, for the first time, Bragg gratings in pure silica core optical fibers. Such optical fibers have demonstrated advantageous resistance to a number of environmental and nonlinear effects, resistance that is often superior to that of doped-core optical fibers, especially commercial doped core optical fibers.